Hybrid MMC low-frequency weak grid network oscillator adaptive rectification control method

Abstract

The application provides a mixed MMC low-frequency weak power grid network oscillator adaptive rectification control method, relates to the power transmission field, and comprises the following steps: constructing a low-frequency network oscillator; filtering the active power on the alternating current side, and determining a frequency adaptive adjustment amount in combination with the power demand on the direct current side; determining a weak power grid stability enhancement adjustment amount according to the current variation, the voltage variation, the voltage deviation, the reactive power deviation, the voltage variation rate and the weak power grid voltage dynamic compensation amount at the low-frequency alternating current side connection point of the converter; extracting a low-frequency oscillation power component from the filtered active power on the alternating current side, obtaining an oscillation power through band-pass filtering, determining an oscillation suppression adjustment amount in combination with the oscillation power dynamic compensation amount; determining a network angle frequency, a network voltage phase angle and a network voltage amplitude, and generating a three-phase low-frequency network voltage reference signal, and the method has the advantages of improving the dynamic adjustment capability and operation coordination of the mixed MMC under the low-frequency weak power grid network rectification operation condition.

Description

Technical Field

[0001] This invention relates to the field of power transmission, and in particular to an adaptive rectification control method for hybrid MMC low-frequency weak grid oscillators. Background Technology

[0002] Modular multilevel converters (MMCs) have been widely used in flexible DC transmission, AC transmission, and renewable energy grid connection due to their significant advantages such as high modularity, good output waveform quality, and suitability for medium- and high-voltage, large-capacity scenarios. With the development of low-frequency AC transmission, weak grid access, and new power system operation modes, the need for MMCs to achieve power conversion and voltage support on the low-frequency AC side is becoming increasingly urgent. Especially in grid-connected rectifier operation scenarios, the converter not only undertakes the energy transmission task from the AC side to the DC side but also needs to have the ability to actively establish AC side voltage and frequency, which places higher demands on grid control performance. In grid-connected operation mode, existing technologies typically use virtual synchronous machine control, droop control, or power synchronization control to enable the converter to form voltage source characteristics and adjust the AC side voltage amplitude and phase based on state variables such as power, voltage, and frequency.

[0003] The existing technology has the following problems: (1) Existing frequency adaptive regulation methods are usually designed around power deviation, frequency deviation, or control parameter correction, which can improve frequency response performance to a certain extent. However, they are mostly used as independent frequency regulation links and do not incorporate low-frequency operating characteristics as regulation components into the frequency dynamic equation of the grid oscillator. It is difficult to make the frequency regulation process and the low-frequency grid voltage generation process form a consistent dynamic relationship. Therefore, there are still certain limitations under low-frequency grid rectification operation conditions.

[0004] (2) Although the existing grid strength sensing and voltage support methods have been applied in some grid control and can characterize the grid state through the grid equivalent impedance or short-circuit ratio, the relevant regulation is usually implemented in the form of parameter correction or independent voltage control links. A unified dynamic regulation path has not been formed, and it is difficult to directly introduce the influence of grid strength changes on the dynamic characteristics of the system into the core model of low-frequency grid control. Therefore, the synergistic effect of frequency and voltage regulation under weak grid conditions is still limited. (3) Regarding the power oscillation problem, existing suppression methods typically achieve oscillation suppression by adding damping elements, filtering compensation elements, or adjusting damping parameters, which can improve the system oscillation response to a certain extent. However, these methods mostly exist as independent compensation channels, and the oscillation state variables are not introduced as unified adjustment components into the frequency dynamic equation and voltage dynamic equation of the grid oscillator, making it difficult to make the damping adjustment process consistent with the grid frequency and voltage dynamic generation process. Therefore, under the conditions of low-frequency weak grid rectifier operation, its adaptive suppression capability and dynamic adjustment coordination for low-frequency power oscillations still have certain limitations.

[0005] (4) Existing methods are mostly designed for a specific control objective in frequency regulation, voltage support or oscillation suppression, and lack a unified coordination mechanism for low-frequency weak power grid rectification operation scenarios. Especially when the system operating frequency, grid strength and power oscillation state change simultaneously, the interaction between multiple control links is difficult to describe and coordinate through a unified model, affecting the consistency and stability of the overall dynamic regulation of the system.

[0006] Therefore, there is a need to provide an adaptive rectification control method for hybrid MMC low-frequency weak grid oscillators to improve the dynamic adjustment capability and system operation coordination of hybrid MMC under low-frequency weak grid rectification operation conditions. Summary of the Invention

[0007] This invention provides an adaptive rectification control method for a hybrid MMC low-frequency weak network oscillator, comprising: constructing a low-frequency network oscillator, wherein the construction of the low-frequency network oscillator includes a network voltage phase angle generation relationship model, a network angular frequency dynamic equation, and a voltage amplitude dynamic adjustment model; collecting state data of the hybrid MMC, wherein the state data includes at least AC-side active power, reactive power, DC-side power demand, and voltage and current at the low-frequency AC-side connection point of the converter; filtering the AC-side active power, obtaining the filtered AC-side active power deviation by combining it with the DC-side power demand, calculating the low-frequency operating factor, adaptive frequency adjustment gain, and frequency adjustment dynamic compensation amount, and determining the frequency adaptive adjustment amount; calculating the equivalent short-circuit ratio estimate and weak network sensitivity based on the current and voltage changes at the low-frequency AC-side connection point of the converter, and combining... The voltage deviation, reactive power deviation, voltage change rate, and dynamic compensation amount of the weak grid voltage at the low-frequency AC side connection point of the converter are used to determine the weak grid stability enhancement regulation amount. The low-frequency oscillation power component is extracted from the filtered AC side active power and obtained by bandpass filtering. Based on the oscillation power, an oscillation energy function is constructed, the adaptive damping regulation coefficient is calculated, and the oscillation suppression regulation amount is determined by combining the dynamic compensation amount of oscillation power. Through a low-frequency grid oscillator, the frequency adaptive regulation amount, the weak grid stability enhancement regulation amount, and the oscillation suppression regulation amount are simultaneously applied to the grid angular frequency dynamic equation. The weak grid stability enhancement regulation amount and the oscillation suppression regulation amount are applied to the voltage amplitude dynamic regulation model to determine the grid angular frequency, the grid voltage amplitude, and the grid voltage phase angle generated by the grid angular frequency, thereby generating a three-phase low-frequency grid voltage reference signal acting on the hybrid MMC.

[0008] Furthermore, the dynamic equation for the mesh angular frequency is: , , in, For virtual inertia coefficient, The rate of change of the angular frequency of the network is This is a reference value for active power. For AC side active power, This is the frequency damping coefficient. For the angular frequency of the mesh, For the target low-frequency angular frequency, This is the phase angle synchronization adjustment coefficient. For the grid voltage phase angle, The equivalent phase angle of the power grid, For comprehensive adjustment, This is the frequency adaptive adjustment amount. To enhance regulation in weak power grids, This is the oscillation suppression adjustment amount.

[0009] Furthermore, the dynamic voltage amplitude adjustment model is as follows: , in, The voltage regulation time constant, The rate of change of the grid voltage amplitude, This serves as a reference value for the grid voltage amplitude. This represents the equivalent amplitude of the AC-side grid voltage. This is the reactive power-voltage regulation coefficient. For AC side reactive power, This is a reference value for reactive power. These are the weighting coefficients. To enhance regulation in weak power grids, This is the damping injection coefficient for the oscillation suppression regulation on voltage amplitude adjustment. This is the oscillation suppression adjustment amount.

[0010] Furthermore, the AC-side active power is filtered, and the filtered AC-side active power deviation is obtained by combining it with the DC-side power demand. The low-frequency operating factor, adaptive frequency adjustment gain, and frequency adjustment dynamic compensation are calculated to determine the adaptive frequency adjustment amount. This includes: dynamically filtering the AC-side active power to generate filtered AC-side active power; generating filtered AC-side active power deviation based on the reference power mapped from the DC-side power demand and the filtered AC-side active power; calculating the dynamic characteristic quantity and integral state quantity of the filtered AC-side active power deviation based on the filtered AC-side active power deviation; calculating the low-frequency operating factor based on the reference angular frequency and the angular frequency output by the grid oscillator; calculating the adaptive frequency adjustment gain based on the low-frequency operating factor; calculating the frequency adjustment dynamic compensation amount based on the filtered AC-side active power deviation and its dynamic characteristic quantity; and calculating the adaptive frequency adjustment amount based on the filtered AC-side active power deviation, its dynamic characteristic quantity and integral state quantity, the adaptive frequency adjustment gain, and the frequency adjustment dynamic compensation amount.

[0011] Furthermore, the frequency adaptive adjustment amount is calculated based on the following formula: , in, This is the frequency adaptive adjustment amount. To adaptively adjust the gain at the frequency, , , and These are the weighting coefficients. This refers to the AC-side active power deviation after filtering. This is the dynamic characteristic quantity of the AC-side active power deviation after filtering. This is the integral state quantity of the AC-side active power deviation after filtering. This is the dynamic compensation amount for frequency adjustment; After introducing the frequency adaptive adjustment, the system frequency adjustment channel is as follows: , , in, For the Laplace transform of the angular frequency deviation of the network, This is the Laplace transform of the filtered AC-side active power deviation. For the Laplace operator, This is the phase angle synchronization adjustment coefficient. For frequency adaptive gain adjustment, For virtual inertia coefficient, This is the frequency damping coefficient. The transfer function of the composite adjustment structure, , The lead compensation time constant, , This is the lag adjustment time constant.

[0012] Furthermore, based on the current and voltage changes at the low-frequency AC side connection point of the converter, the equivalent short-circuit ratio estimate and the weak grid sensitivity are calculated. Combining the voltage deviation, reactive power deviation, voltage change rate at the low-frequency AC side connection point of the converter, and the weak grid voltage dynamic compensation, the weak grid stability enhancement regulation is determined. This includes: calculating the grid equivalent impedance based on the current and voltage changes at the low-frequency AC side connection point of the converter using an estimation model of the grid equivalent impedance; calculating the equivalent short-circuit ratio estimate based on the grid equivalent impedance; calculating the weak grid sensitivity based on the equivalent short-circuit ratio estimate; calculating the weak grid voltage dynamic compensation based on the voltage deviation and voltage change rate at the low-frequency AC side connection point of the converter; and calculating the weak grid stability enhancement regulation based on the voltage deviation, reactive power deviation, voltage change rate at the low-frequency AC side connection point of the converter, and the weak grid sensitivity.

[0013] Furthermore, the stability enhancement regulation of the weak power grid is calculated based on the following formula: , in, To enhance regulation in weak power grids, To enhance adaptive gain for weak grid stability, , , and These are the weighting coefficients. This refers to the voltage deviation at the low-frequency AC side connection point of the converter. This refers to reactive power deviation. The rate of change of voltage. This is the dynamic voltage compensation amount for weak power grids; After introducing frequency adaptive regulation and weak grid stability enhancement regulation, the system frequency regulation channel is represented as follows: , , in, The equivalent transfer function for the stability enhancement channel in a weak power grid is... To enhance adaptive gain for weak grid stability, This is the advance compensation time constant; The main dynamic time constant; is the filter damping time constant.

[0014] Furthermore, the low-frequency oscillation power component is extracted from the filtered AC-side active power, and the oscillation power is obtained by bandpass filtering. An oscillation energy function is constructed based on the oscillation power, and an adaptive damping adjustment coefficient is calculated. Combined with the dynamic compensation amount of the oscillation power, the oscillation suppression adjustment amount is determined, including: calculating the average power component based on the filtered AC-side active power; calculating the low-frequency oscillation power component based on the filtered AC-side active power and the average power component; generating the bandpass-filtered oscillation power based on the low-frequency oscillation power component using a second-order bandpass filter; calculating the oscillation energy based on the bandpass-filtered oscillation power; calculating the adaptive damping adjustment coefficient based on the oscillation energy; calculating the dynamic compensation amount of the oscillation power based on the low-frequency oscillation power component and its rate of change; and determining the oscillation suppression adjustment amount based on the dynamic compensation amount of the oscillation power, the adaptive damping adjustment coefficient, the low-frequency oscillation power component, and its rate of change.

[0015] Furthermore, the oscillation suppression adjustment amount is calculated based on the following formula: , in, This is the oscillation suppression adjustment amount. , and These are the weighting coefficients. This is the adaptive damping adjustment coefficient. This is the low-frequency oscillation power component. The rate of change of the low-frequency oscillation power component. This is the dynamic compensation amount for oscillation power; After introducing frequency adaptive adjustment, weak grid stability enhancement adjustment, and oscillation suppression adjustment, the system frequency adjustment channel is expressed as follows: , , in, This is the adaptive damping adjustment coefficient. This is the equivalent transfer function of the low-frequency power oscillation suppression control channel; , This is the phase lead compensation time constant. , This is the phase lag compensation time constant.

[0016] Furthermore, by using a low-frequency grid oscillator, the frequency adaptive adjustment, the weak grid stability enhancement adjustment, and the oscillation suppression adjustment are simultaneously applied to the grid angular frequency dynamic equation, and the weak grid stability enhancement adjustment and oscillation suppression adjustment are simultaneously applied to the voltage amplitude dynamic adjustment model to determine the grid angular frequency, the grid voltage amplitude, and the grid voltage phase angle generated by the grid angular frequency. This process then generates a three-phase low-frequency grid voltage reference signal acting on the hybrid MMC, including: determining the grid voltage amplitude based on the weak grid stability enhancement adjustment and the oscillation suppression adjustment using the voltage amplitude dynamic adjustment model; determining the grid angular frequency based on the frequency adaptive adjustment, the weak grid stability enhancement adjustment, and the oscillation suppression adjustment using the grid angular frequency dynamic equation; determining the grid voltage phase angle based on the grid angular frequency using the grid voltage phase angle generation relationship model; and generating a three-phase low-frequency grid voltage reference signal acting on the hybrid MMC based on the grid voltage phase angle and the grid voltage amplitude.

[0017] Compared with existing technologies, the adaptive rectification control method for hybrid MMC low-frequency weak grid oscillators provided by this invention has at least the following advantages: By constructing a low-frequency grid oscillator and establishing a dynamic adjustment relationship between frequency and voltage, the grid voltage was transformed from reference-driven to dynamically adjusted according to power state, significantly improving the dynamic coordination capability between frequency and voltage. Simultaneously, a low-frequency operating characteristic factor was introduced to construct a frequency adaptive adjustment mechanism, solving the problems of slow frequency response and unstable adjustment under low-frequency operation.

[0018] By estimating the equivalent impedance of the power grid online and constructing a power grid strength-driven mechanism, the voltage support and dynamic adaptability under weak power grid conditions are enhanced.

[0019] Furthermore, by constructing an oscillation power component extraction and energy function to achieve adaptive adjustment of the damping coefficient, low-frequency power oscillations are effectively suppressed; and frequency adjustment, weak grid stability enhancement and oscillation suppression are uniformly coupled under the grid oscillator framework, avoiding the amplification of oscillations between different control channels.

[0020] Ultimately, the dynamic adjustment involves the combined participation of low-frequency operating characteristics, grid strength changes, and oscillation states, significantly improving the frequency response, voltage support, and power oscillation suppression in low-frequency weak grid rectifier operation, and greatly enhancing the overall operational stability and adaptability of the hybrid MMC. Attached Figure Description

[0021] This specification will be further described by way of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not limiting; in these embodiments, the same reference numerals denote the same structures, wherein: Figure 1 This is a flowchart illustrating the adaptive rectification control method for a hybrid MMC low-frequency weak grid oscillator according to some embodiments of this specification. Figure 2 These are network frequency response waveforms shown in some embodiments of this specification; Figure 3 These are DC-side voltage waveform diagrams shown according to some embodiments of this specification; Figure 4 These are AC side voltage waveform diagrams shown according to some embodiments of this specification; Figure 5 This is a partially enlarged waveform diagram of the AC side voltage shown in some embodiments of this specification; Figure 6 These are AC side current waveform diagrams shown according to some embodiments of this specification; Figure 7 This is a partially magnified waveform diagram of the AC side current shown in some embodiments of this specification; Figure 8 This is an AC side input power curve diagram shown according to some embodiments of this specification; Figure 9 This is a DC-side output power curve shown according to some embodiments of this specification; Figure 10 This is a waveform diagram of the internal circulation of a hybrid MMC as shown in some embodiments of this specification; Figure 11 This is a graph showing the voltage variation of the submodule capacitor according to some embodiments of this specification; Figure 12 This is a partially enlarged view of the capacitor voltage of a submodule shown in some embodiments of this specification; Figure 13 This is a control block diagram shown according to some embodiments of this specification. Detailed Implementation

[0022] To more clearly illustrate the technical solutions of the embodiments in this specification, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are merely some examples or embodiments of this specification. For those skilled in the art, these drawings can be applied to other similar scenarios without creative effort. Unless obvious from the context or otherwise specified, the same reference numerals in the drawings represent the same structures or operations.

[0023] Figure 1 This is a flowchart illustrating the adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to some embodiments of this specification, such as... Figure 1 As shown, the adaptive rectification control method for hybrid MMC low-frequency weak grid oscillators may include the following steps.

[0024] S1. Construct a low-frequency grid oscillator.

[0025] like Figure 13 As shown, the construction of a low-frequency grid oscillator includes a grid voltage phase angle generation relationship model, a grid angular frequency dynamic equation, and a voltage amplitude dynamic adjustment model.

[0026] Specifically, in a hybrid modular multilevel converter grid rectifier system operating under low-frequency, weak grid conditions, the converter achieves rectification by absorbing active power from the AC side and transferring energy to the DC side, with the power flow direction opposite to that of a traditional grid inverter mode. Under this operating mode, there is a significant dynamic relationship between AC side frequency and voltage and DC side power. Especially under low-frequency operating conditions, due to the extended power exchange cycle, reduced system equivalent inertia, and enhanced influence of weak grid impedance, power fluctuations during rectification can easily trigger interactive oscillations between frequency and voltage. Existing grid control methods based on fixed frequency references or single power deviation adjustment struggle to simultaneously address both frequency dynamic response and voltage support capabilities.

[0027] Constructing the state variable system of a low-frequency grid oscillator, including grid voltage and phase angle. Angular frequency of the network Equivalent voltage amplitude and AC active power and reactive power .in, , and These are used to describe the phase evolution, frequency change, and amplitude regulation process of the grid voltage, respectively. This represents the active power absorbed by the converter from the AC side during rectifier operation. Its magnitude reflects the energy balance between the AC side input power and the DC side power. Reactive power represents reactive power, which is used in the voltage regulation process. Active power and reactive power are expressed as follows: , , in, , It is three-phase voltage , , After Clarke transform, Two orthogonal components in a stationary coordinate system for Axis current components, for Axial current component.

[0028] The model for generating the phase angle of the grid voltage is as follows: , in, This represents the rate of change of the grid voltage phase angle. This grid voltage phase angle generation model is used to achieve continuous generation of the grid voltage phase and is the fundamental dynamic relationship of the grid oscillator.

[0029] Unlike existing network configurations that rely solely on a fixed frequency reference, this method introduces a unified adjustment mechanism for power deviation, frequency deviation, and phase angle deviation in the frequency dynamic equation, and considers the dynamic imbalance between AC-side input power and DC-side power under rectified operation conditions.

[0030] In some embodiments, the dynamic equation for the mesh angular frequency is: , , in, For virtual inertia coefficient, The rate of change of the angular frequency of the network is This is a reference value for active power. For AC side active power, This is the frequency damping coefficient. For the angular frequency of the mesh, For the target low-frequency angular frequency, This is the phase angle synchronization adjustment coefficient. For the grid voltage phase angle, The equivalent phase angle of the power grid, For comprehensive adjustment, This is the frequency adaptive adjustment amount. To enhance regulation in weak power grids, This is the oscillation suppression adjustment amount.

[0031] By using the above-mentioned dynamic equation for the angular frequency of the grid, the frequency regulation process can simultaneously reflect the power transmission relationship and frequency deviation, thereby realizing the coordinated regulation of multiple states for rectified operation.

[0032] In some embodiments, regarding voltage regulation, to avoid the insufficient support capacity of the traditional single voltage-reactive power control method under weak grid and rectifier operation conditions, a dynamic voltage amplitude regulation model is constructed as follows: , in, The voltage regulation time constant, The rate of change of the grid voltage amplitude is used to characterize the dynamic process of voltage regulation. This serves as a reference value for the grid voltage amplitude. This represents the equivalent amplitude of the AC-side grid voltage. This is the reactive power-voltage regulation coefficient. For AC side reactive power, This is a reactive power reference value used to adjust the reactive power output level and voltage support capability of the converter. The weighting coefficient for voltage amplitude regulation by the regulation quantity enhancing stability in weak power grids. To enhance regulation in weak power grids, This is the damping injection coefficient for the oscillation suppression regulation on voltage amplitude adjustment. This is the oscillation suppression adjustment value. As an example, in a grid rectifier system with a target low-frequency operating frequency of 20Hz, a value of [value] can be taken as [value]. This value is slightly larger than a 20Hz low-frequency cycle. This allows for a certain degree of dynamic smoothness in voltage amplitude regulation, preventing overshoot or oscillation caused by excessively rapid voltage amplitude response. (This is a recommended approach.) This value allows the voltage regulation enhancement amount to appropriately participate in voltage amplitude support when the grid strength is reduced, thus strengthening the voltage regulation effect, while avoiding excessive weighting that could lead to overly strong voltage amplitude regulation. A suitable value is... This value enables the oscillation suppression adjustment to provide auxiliary damping for the voltage amplitude dynamics, weakening the impact of low-frequency power oscillations on the voltage amplitude, while avoiding excessive damping injection that could lead to a slower voltage response or suppression of power dynamics.

[0033] The above-mentioned dynamic voltage amplitude adjustment model enables dynamic adjustment of voltage amplitude according to changes in reactive power and external adjustment quantities.

[0034] To achieve unified coordination among multiple control links, the outputs of each subsequent control branch are uniformly represented as a comprehensive adjustment quantity. : , in, This is a low-frequency adaptive adjustment amount. To enhance regulation in weak power grids, This is the regulation amount for suppressing low-frequency power oscillations. The comprehensive regulation amount acts simultaneously on the frequency dynamic equation and participates in voltage regulation through the regulation component in the voltage dynamic equation, thereby achieving a synergistic effect between frequency regulation and voltage regulation.

[0035] Based on the amplitude of the grid voltage output by the low-frequency grid oscillator Phase angle with grid voltage Generate a three-phase low-frequency grid voltage reference signal: , , , in, , and This is the reference signal for the three-phase low-frequency grid voltage.

[0036] The three-phase low-frequency grid voltage reference signal serves as the reference input for converter modulation control, generating corresponding modulation commands to achieve autonomous establishment of AC side voltage under grid rectification conditions.

[0037] To further analyze the dynamic characteristics of the constructed low-frequency grid oscillator under rectified operation conditions, small-signal linearization of the system variables is performed near the steady-state operating point. Let the angular frequency and active power of the system during steady-state operation be respectively... and Then the disturbance of each variable is defined as: , , , in: This refers to the change in the network angular frequency relative to the steady-state angular frequency. This represents the change in AC active power relative to steady-state active power. This represents the change in reference power relative to steady-state active power, obtained by mapping DC-side power demand.

[0038] Applying a Laplace transform to the above perturbation yields its corresponding frequency domain expression. Based on this, from the perspective of system dynamics, the fundamental frequency adjustment channel of the constructed low-frequency grid oscillator in the Laplace domain can be expressed as: , This expression shows that the constructed grid oscillator not only includes inertial response and damping regulation characteristics, but also participates in dynamic frequency regulation by introducing phase angle deviation to achieve dynamic synchronization with the power grid. Furthermore, it introduces dynamic compensation of multiple control links through comprehensive regulation, thereby achieving stable frequency regulation capability under low-frequency weak power grid and rectifier operation conditions.

[0039] The low-frequency grid oscillator constructed by this method transforms the grid voltage generation process from the traditional fixed reference driving mode to a frequency and voltage coordinated dynamic adjustment mode oriented towards rectification operation. Furthermore, it achieves unified injection of multiple control links through a comprehensive adjustment interface, thereby providing basic support for grid rectification control under low-frequency weak grid conditions.

[0040] S2. Collect status data of hybrid MMC.

[0041] Specifically, the status data of a hybrid MMC can include AC side active power, reactive power, DC side power demand, and voltage and current at the low-frequency AC side connection point of the converter.

[0042] S3. Filter the active power on the AC side, and obtain the deviation of the active power on the AC side after filtering by combining the power demand on the DC side. Calculate the low-frequency operating factor, adaptive frequency adjustment gain and frequency adjustment dynamic compensation amount, and determine the adaptive frequency adjustment amount.

[0043] like Figure 13 As shown, it specifically includes: The active power on the AC side is dynamically filtered to generate filtered active power on the AC side. Based on the reference power obtained by mapping the DC side power demand and the filtered AC side active power, a filtered AC side active power deviation is generated. Based on the filtered AC-side active power deviation, the dynamic characteristic quantity and integral state quantity of the filtered AC-side active power deviation are calculated. Calculate the low-frequency operating factor based on the reference angular frequency and the angular frequency output by the grid oscillator; Calculate the adaptive frequency adjustment gain based on the low-frequency operating factor; Based on the filtered AC-side active power deviation and the dynamic characteristic quantity of the filtered AC-side active power deviation, the dynamic compensation quantity of frequency regulation is calculated. The adaptive frequency adjustment is calculated based on the filtered AC-side active power deviation, the dynamic characteristic quantity and integral state quantity of the filtered AC-side active power deviation, the adaptive frequency adjustment gain, and the frequency adjustment dynamic compensation quantity.

[0044] Specifically, the regulation of system frequency is mainly driven by active power imbalance. However, under the conditions of low-frequency weak grid rectification operation, due to the extended power exchange cycle on the AC side and the dynamic changes in power demand on the DC side, the active power deviation on the AC side of the system exhibits low-frequency and slowly changing characteristics. Based on existing frequency regulation methods, it is difficult to simultaneously take into account dynamic response speed and stability, which can easily lead to slow frequency recovery or oscillation amplification.

[0045] This method introduces a low-frequency operating state factor and a multi-state dynamic compensation structure, enabling the frequency regulation capability to adaptively adjust with changes in the system's operating state, thereby improving the frequency stability under low-frequency grid rectification operating conditions.

[0046] Under the conditions of grid rectification operation, the active power deviation on the AC side after filtering is used as a characterization of the dynamic change of system power. It is used to reflect the impact of the power transmission process on the frequency regulation process and participates in the subsequent dynamic compensation calculation as the input of the frequency adaptive regulation channel.

[0047] Dynamic filtering of AC-side active power includes: , in, This represents the filtered AC-side active power. This represents the rate of change of the filtered AC-side active power. This represents the active power filtering time constant. For example, when the system's target low-frequency operating frequency is 20Hz, the active power filtering time constant is... Desirable This value is less than one low-frequency operating cycle, which can smooth the fluctuation components in active power measurement, while avoiding excessive filtering lag from affecting the dynamic response of the frequency adaptive adjustment.

[0048] Further define the filtered AC-side active power deviation for: , in, This is the reference power obtained by mapping the DC-side power demand.

[0049] Dynamic characteristic quantity of AC side active power deviation after filtering for: .

[0050] Integral state quantity of the AC-side active power deviation after filtering for: , in, This represents the instantaneous value of the AC-side active power deviation after filtering at time τ.

[0051] The filtered AC-side active power deviation, the dynamic characteristic quantity of the filtered AC-side active power deviation, and the integral state quantity of the filtered AC-side active power deviation describe the amplitude, trend, and cumulative effect of the rectified power imbalance, respectively, providing multi-dimensional dynamic information for frequency regulation.

[0052] To reflect the impact of low-frequency operation on frequency regulation capability, a low-frequency operation factor is introduced. : , in, As the reference angular frequency, The gridding angular frequency output by the low-frequency gridding oscillator. To prevent the division by tiny positive numbers with a denominator of zero, when the system is operating at low frequencies, reduce, This increases the frequency, thereby enhancing the response to power deviations and enabling adaptive enhancement of frequency regulation capabilities as the operating conditions decrease. For example, when the target operating frequency of the low-frequency AC side of the hybrid MMC is 20Hz, a reference angular frequency can be used. Under steady-state low-frequency operation, the mesh angular frequency Approximately At this time, the low-frequency operating factor Approximately 1; when the system frequency is lower than the target operating frequency, Increase the frequency, thereby enhancing the adaptive frequency adjustment function.

[0053] Adaptive frequency-adjustable gain for: , in, To obtain the base gain under rated conditions, the filtered AC-side active power deviation, its dynamic characteristics, and integral state parameters are normalized to per unit based on the system's rated capacity and reference angular frequency. The value can be 0.05–0.20, with 0.08–0.15 being preferred. This is the low-frequency sensitivity adjustment coefficient, used to adjust the low-frequency operating factor. Frequency adaptive gain The correction strength can be determined through small-signal analysis and time-domain simulation based on the system's low-frequency operating frequency, rated frequency, converter capacity level, and target dynamic response indicators. The preferred value is 0.2 to 0.5, in order to balance the frequency adjustment sensitivity and system dynamic stability under low-frequency operating conditions.

[0054] Unlike traditional parameter-based frequency control methods, this method enables the frequency adjustment gain to be dynamically adjusted according to changes in the system's operating frequency, thereby improving the adjustment sensitivity under low-frequency operating conditions.

[0055] Based on this, to overcome the problem of insufficient dynamic performance of traditional PI or droop control under low-frequency conditions, a composite regulation structure containing multiple dynamic compensation elements is constructed, whose Laplace domain expression is: , in, The transfer function of the composite adjustment structure, , The lead compensation time constant, , For the lag adjustment time constant, for example, if the target low-frequency operating frequency of this system is 20Hz, and the control quantity is normalized, it can be taken as... , , , .

[0056] This structure eliminates steady-state error through an integral element, enhances phase margin under low-frequency disturbances through a double lead element, and suppresses high-frequency noise amplification through a lag element, thereby achieving comprehensive optimization of frequency regulation performance under low-frequency conditions.

[0057] To improve the system's dynamic response under low-frequency conditions, a multi-state variable collaborative adjustment structure is constructed, and a frequency regulation dynamic compensation variable is introduced. : , in, , These are the adjustment time constants of the compensation channels, Dynamic compensation amount for frequency adjustment The rate of change, for example, the target low-frequency operating frequency of this system is 20Hz, and the active power deviation on the AC side after filtering is normalized, can be taken as... , At this point, the first dynamic compensation quantity satisfies... This value can provide appropriate phase lead compensation under low-frequency power disturbance conditions, improve the dynamic response capability of the frequency adjustment channel, and avoid excessive compensation leading to frequency overshoot or oscillation amplification.

[0058] By introducing dynamic compensation for frequency regulation, additional phase correction can be provided under low-frequency disturbance conditions, so that the frequency regulation not only reflects the magnitude of power error, but also the trend of power deviation change and the dynamic advance demand of the system.

[0059] In some embodiments, the frequency adaptive adjustment amount is calculated based on the following formula: , in, This is the frequency adaptive adjustment amount. To adaptively adjust the gain at the frequency, , , and These are the weighting coefficients. Used to adjust compensation state variables The strength of its role in frequency adaptive regulation This refers to the AC-side active power deviation after filtering. This is a dynamic characteristic quantity of the AC-side active power deviation after filtering. This is the integral state quantity of the AC-side active power deviation after filtering. This is the dynamic compensation amount for frequency adjustment. For example, when the system's target low-frequency operating frequency is 20Hz, and the filtered AC-side active power deviation and related state variables are standardized, it can be taken as... , , , The above parameters are equivalent weighting coefficients under the per-unit control model, used to adjust the strength of the effects of the filtered AC-side active power deviation, power deviation rate of change, integral state quantity, and dynamic compensation quantity in frequency adaptive regulation.

[0060] After introducing the frequency adaptive adjustment, the system frequency adjustment channel is as follows: , , in, For the Laplace transform of the angular frequency deviation of the network, This is the Laplace transform of the filtered AC-side active power deviation. For the Laplace operator, This is the phase angle synchronization adjustment coefficient. For frequency adaptive gain adjustment, For virtual inertia coefficient, This is the frequency damping coefficient. The transfer function of the composite adjustment structure, , The lead compensation time constant, , This represents the lag adjustment time constant. As an example, if the system's target low-frequency operating frequency is 20Hz and the control input is standardized, then can be taken as... , , , .

[0061] Introducing a frequency adaptive adjustment can, on the one hand, improve the dynamic adjustment speed of the grid frequency and shorten the frequency recovery time; on the other hand, it can suppress frequency overshoot and low-frequency oscillation caused by fixed parameter adjustment, thereby providing a more stable frequency basis for the weak grid stability enhancement control and low-frequency power oscillation suppression control in subsequent steps.

[0062] S4. Based on the current and voltage changes at the low-frequency AC side connection point of the converter, calculate the equivalent short-circuit ratio estimate and the weak grid sensitivity. Combine the voltage deviation, reactive power deviation, voltage change rate, and weak grid voltage dynamic compensation at the low-frequency AC side connection point of the converter to determine the weak grid stability enhancement regulation.

[0063] like Figure 13 As shown, S4 specifically includes: The equivalent impedance of the power grid is calculated based on the current and voltage changes at the low-frequency AC side connection point of the converter using the estimation model of the equivalent impedance of the power grid. Calculate the equivalent short-circuit ratio estimate based on the power grid equivalent impedance; Calculate the sensitivity of the weak power grid based on the equivalent short-circuit ratio estimate; Calculate the dynamic compensation amount for weak grid voltage based on the voltage deviation and voltage change rate at the low-frequency AC side connection point of the converter. The regulation amount for enhancing stability of the weak power grid is calculated based on the voltage deviation, reactive power deviation, voltage change rate, and weak power grid sensitivity at the low-frequency AC side connection point of the converter.

[0064] Specifically, under low-frequency weak grid rectification operation conditions, due to reduced grid short-circuit capacity, increased equivalent impedance, and enhanced dynamic correlation in the power transmission process between the AC and DC sides, the system becomes more sensitive to voltage disturbances and power fluctuations, easily leading to problems such as amplified voltage fluctuations, unstable power transmission, and frequency-voltage oscillations. While existing weak grid voltage support and impedance regulation methods can characterize grid strength changes by incorporating information such as grid equivalent impedance and short-circuit ratio, and adjust parameters or add control accordingly, these adjustments are mostly treated as independent voltage control or reactive power regulation components. Grid strength information has not yet been incorporated as a unified regulation component into the dynamic equation of the grid angular frequency and the dynamic voltage amplitude regulation model of the low-frequency grid oscillator. Therefore, under low-frequency weak grid rectification operation conditions, the impact of grid state changes on the grid frequency and voltage dynamic regulation process is difficult to coordinate and reflect within the same control model, thus limiting the system's voltage support capability and dynamic stability under weak grid conditions.

[0065] First, an estimation model for the equivalent impedance of the power grid is constructed.

[0066] Define the changes in voltage and current at the low-frequency AC side connection point of the converter as follows: , , in, This refers to the voltage change at the low-frequency AC side connection point of the converter. This refers to the change in current at the low-frequency AC side connection point of the converter. and These are the real-time voltage amplitude and real-time current amplitude at the low-frequency AC side connection point of the converter, respectively. and These are the reference values ​​for the voltage and current amplitudes at the low-frequency AC side connection points of the converter.

[0067] The estimation model for the equivalent impedance of the power grid is as follows: , in, It represents the rate of change of the estimated equivalent impedance of the power grid, and is used to describe the dynamic process of the power grid impedance changing with the operating state. To estimate the obtained equivalent impedance of the power grid, To estimate the filter time constant for impedance estimation, To prevent the use of tiny positive numbers with a denominator of zero, this model can reflect changes in grid impedance in real time, thus providing dynamic information on grid strength for subsequent control. Impedance estimation filtering time constant. The setting can be adjusted based on the sampling period of the low-frequency AC side voltage and current, and the dynamic response requirements for impedance estimation. When the target low-frequency operating frequency of the system is 20Hz, the setting can be adjusted accordingly. This allows the impedance estimation process to be dynamically smoothed within a low-frequency operating cycle, thus balancing estimation stability and response speed.

[0068] Based on the equivalent impedance of the power grid, a weak power grid strength index is introduced. An estimate of the equivalent short-circuit ratio is defined. for: , in, Rated voltage, This refers to the system's rated capacity.

[0069] To enhance sensitivity to changes in the condition of weak power grids, a weak power grid sensitivity feature is further introduced. : , in, This parameter is used to limit the amplification of the weak grid characteristic function and adjust the sensitivity of the controller parameters to changes in grid strength. It can be determined through simulation tuning based on the system's rated short-circuit ratio range, grid impedance variation range, and target voltage dynamic response index. The value must satisfy... The optimal value is 0.1 to 0.3 to balance the sensitivity of weak grid condition sensing and the dynamic stability of the system.

[0070] When the power grid is weaker The smaller, To ensure that changes in grid strength can be directly mapped to the weak grid stability enhancement control, a weak grid stability enhancement adaptive gain is introduced: , in, To enhance adaptive gain for weak grid stability, This is the reference gain coefficient for weak power grids. When the power grid strength decreases... Increase, make This increases accordingly, thereby enhancing voltage support and stability regulation under weak grid conditions. This automatically enhances the subsequent weak grid stability enhancement regulation. Therefore, weak grid sensitivity can map grid strength changes to controller gain adjustment, achieving an adaptive mapping relationship between grid state and control parameters. When the voltage deviation, reactive power deviation, and voltage change rate at the converter's low-frequency AC side connection point are all normalized, the weak grid enhancement reference gain coefficient... Desirable This value provides a moderate boost to stability in weak power grids while avoiding excessive gain that could lead to voltage regulation overshoot or oscillation amplification.

[0071] Under grid-connected rectification operation conditions, voltage disturbances not only affect the AC side voltage stability, but also affect the DC side power stability through the power transfer process. (Definition:) , , in, This represents the deviation between the reference voltage value at the low-frequency AC side connection point of the converter and the actual voltage at the same connection point; in other words, the voltage deviation at the low-frequency AC side connection point of the converter. This is a reference value for the low-frequency AC side voltage. This indicates reactive power deviation.

[0072] Voltage change rate for: .

[0073] To improve the system's dynamic response capability under weak grid conditions, a weak grid voltage dynamic compensation quantity is introduced. : , in, The rate of change of the dynamic voltage compensation quantity in a weak power grid. and The time constant of the compensation element is used to adjust the system's dynamic response characteristics to voltage disturbances and their changing trends. For example, when the system's target low-frequency operating frequency is 20Hz, and the voltage deviation at the converter's low-frequency AC side connection point is normalized, the time constant of the compensation element... Desirable , Desirable .

[0074] In some embodiments, the stability enhancement regulation for weak power grids is calculated based on the following formula: , in, To enhance regulation in weak power grids, , , and These are the weighting coefficients. This refers to the voltage deviation at the low-frequency AC side connection point of the converter. This refers to reactive power deviation. The rate of change of voltage. This is the dynamic voltage compensation value for a weak power grid. It can be taken as... , , , The voltage change term serves as the primary regulating variable, while the reactive power deviation term and voltage change rate term serve as auxiliary correction terms. The dynamic voltage compensation term for weak grids is used to improve the dynamic response under weak grid disturbances. This value can enhance the voltage support under weak grid conditions, while avoiding excessive voltage change rate and compensation terms that could lead to noise amplification or voltage oscillations.

[0075] After introducing frequency adaptive regulation and weak grid stability enhancement regulation, the system frequency regulation channel is represented as follows: , , in, The equivalent transfer function for the stability enhancement channel in a weak power grid is... To enhance adaptive gain for weak grid stability, This is the advance compensation time constant; The main dynamic time constant; is the filter damping time constant. Determine the intensity of advance compensation (to expedite response); Determines the system's active dynamic speed; Determines the filtering / damping characteristics (high-frequency suppression). Possible... , , .in, Slightly larger It can provide appropriate advance compensation to improve the dynamic response speed of the weak grid stability enhancement channel; To determine the primary adjustment timescale of this channel, take It can balance response speed and stability; Pick It is used to suppress high-frequency disturbances introduced by voltage and current sampling and differentiation, and to prevent high-frequency noise from being amplified.

[0076] This expression shows that weak grid stability enhancement control, by introducing weak grid stability enhancement adjustment, enables the system damping characteristics to adaptively adjust with changes in grid state.

[0077] By introducing a multi-variable collaborative adjustment mechanism for voltage deviation, reactive power deviation and their dynamic changes, and combining it with compensation state variables to construct a dynamic phase compensation channel, the system can simultaneously take into account dynamic response speed and stability under weak grid conditions.

[0078] S5. Extract the low-frequency oscillation power component from the filtered AC active power, and obtain the oscillation power through bandpass filtering. Construct the oscillation energy function based on the oscillation power, calculate the adaptive damping adjustment coefficient, and determine the oscillation suppression adjustment amount by combining the dynamic compensation amount of the oscillation power.

[0079] like Figure 13 As shown, S5 specifically includes: The average power component is calculated based on the filtered AC active power. The low-frequency oscillation power component is calculated based on the filtered AC side active power and average power component. The bandpass filtered oscillation power is generated based on the low-frequency oscillation power component using a second-order bandpass filter. Calculate the oscillation energy based on the oscillation power after bandpass filtering; Calculate the adaptive damping adjustment coefficient based on the oscillation energy; Calculate the dynamic compensation amount of oscillation power based on the low-frequency oscillation power component and the rate of change of the low-frequency oscillation power component; The oscillation suppression adjustment amount is determined based on the dynamic compensation amount of oscillation power, the adaptive damping adjustment coefficient, the low-frequency oscillation power component, and the rate of change of the low-frequency oscillation power component.

[0080] Specifically, due to the extended power exchange cycle, insufficient system inertial support, and enhanced interaction with weak grids under low-frequency grid rectification operation conditions, the system is still prone to low-frequency active power oscillations and frequency fluctuations when subjected to disturbances. Unlike existing power oscillation suppression methods based on fixed damping coefficients, this method achieves adaptive adjustment of the damping coefficient by extracting the low-frequency oscillation power component and constructing an oscillation energy feedback mechanism.

[0081] First, calculate the average power component: , in, The rate of change of average power Average power, The average filter time constant is, as an example, when the target low-frequency operating frequency of the system is 20Hz. Desirable This value is approximately two low-frequency operating cycles, which can be used to determine the effective power after filtering. The smoother average power component is extracted, while the low-frequency oscillation power component is retained for subsequent oscillation energy calculation and adaptive damping adjustment.

[0082] Define the low-frequency oscillation power component as: , middle, This represents the low-frequency oscillation power component.

[0083] To further enhance the ability to identify the target's low-frequency oscillation band, a second-order bandpass filter is introduced. The power component of low-frequency oscillations is selectively extracted, and its Laplace expression is as follows: , in, Let be the transfer function of a second-order bandpass filter. The target low-frequency oscillation angular frequency, is the damping coefficient.

[0084] The oscillation power after bandpass filtering is: , in, This represents the oscillation power after bandpass filtering.

[0085] The above processing enables this method to adjust for low-frequency oscillation components, thereby avoiding interference from non-target frequency band signals on the control effect.

[0086] The oscillation energy function is: , in, For oscillating energy, The rate of change of oscillation power; This is a time scale coefficient used to adjust the weight of the oscillation rate of change in the energy function. In low-frequency, weak-grid rectifier systems, the time scale coefficient of the oscillation energy function... The tuning can be adjusted based on the target oscillation frequency band and the sensitivity to power change rate. When the target low-frequency operating frequency of the system is 20Hz... Pick This value allows the oscillation power change rate term to participate appropriately in the oscillation energy calculation, reflecting the trend of power oscillation changes while avoiding excessive weighting of the change rate term, which could lead to noise amplification or overly sensitive damping adjustment.

[0087] The oscillation energy function can be composed of the oscillation power component and its rate of change, so that it can reflect both the magnitude of the oscillation and the trend of oscillation.

[0088] Adaptive damping adjustment coefficient for: , in, The basic damping coefficient is used to characterize the basic damping level of the system in a state without significant oscillations. (Parameter) The value of should be matched with the damping requirements under the rated operating conditions of the system. Its value is generally positive and should not be too large, so as to avoid excessive suppression of the normal operating state of the system. The value can be 0.1 to 5; preferably 0.5 to 2, to ensure that the system still has appropriate damping characteristics under conditions of no oscillation or small disturbance. This is the oscillation energy adjustment coefficient, used to adjust the oscillation energy. The adaptive amplification effect of the damping coefficient, parameters The value used to adjust the sensitivity of oscillation energy to damping enhancement should be tuned based on the amplitude range of the system's low-frequency oscillations and the target oscillation decay rate. When When the value is small, the damping adjustment is relatively smooth; when When the value is large, the system is more sensitive to changes in oscillation energy, but too large a value may cause the damping coefficient to change too quickly, thus affecting the dynamic stability of the system. The value satisfies The optimal value is 0.2 to 1.0. The above function allows the damping coefficient to be dynamically adjusted according to the oscillation energy: when the oscillation increases, the damping is increased; when the oscillation decreases, the damping is decreased, thereby enhancing the oscillation suppression capability.

[0089] By introducing a dynamic compensation amount for oscillation power, the low-frequency oscillation power component and its rate of change are dynamically compensated, thereby improving the phase characteristics of the damping adjustment channel. Its dynamics satisfy: , in, To compensate for the time constant of the state variables, The time scale coefficient for the oscillatory rate of change term. Represents the rate of change of the compensating state variable. This represents the rate of change of the low-frequency oscillation power component. The above parameters together constitute a dynamic adjustment channel with phase compensation characteristics. Desirable , Desirable .in, Used to determine compensation state variables Dynamic smoothing speed, This value is used to adjust the compensation strength of the oscillation power change rate term. It provides appropriate phase compensation during low-frequency power oscillations while avoiding oversensitivity to the power change rate, which could amplify measurement noise or cause damping adjustment oscillations.

[0090] In some embodiments, the oscillation suppression adjustment amount is calculated based on the following formula: , in, This is the oscillation suppression adjustment amount. , and These are the weighting coefficients. This is the adaptive damping adjustment coefficient. This is the low-frequency oscillation power component. The rate of change of the low-frequency oscillation power component. This is the dynamic compensation value for oscillation power. It can be taken as... , , .in, This is used to ensure that the oscillation power component plays a major role in damping regulation. It is used to reflect the trend of oscillation power change, but the value should not be too large to avoid amplifying noise in the power change rate. Used to adjust compensation state variables The strength of the participation. This value can provide moderate oscillation suppression and avoid excessive damping compensation that could slow down the power dynamic response or generate new oscillations.

[0091] After introducing frequency adaptive adjustment, weak grid stability enhancement adjustment, and oscillation suppression adjustment, the system frequency adjustment channel is expressed as follows: , , in, This is the adaptive damping adjustment coefficient. This is the equivalent transfer function of the low-frequency power oscillation suppression control channel; , This is the phase lead compensation time constant. , This is the phase lag compensation time constant, used to adjust the dynamic response characteristics and stability of the control channel. (Option 1) , , , . , Used to provide phase lead compensation and enhance the oscillation suppression channel's response to low-frequency power oscillation changes; , This is used to provide hysteresis filtering and damping regulation, suppressing high-frequency disturbances in power rate of change signals and bandpass filter outputs. This value balances phase compensation, damping, and noise immunity during low-frequency power oscillation suppression, avoiding excessive compensation that could lead to power dynamic response oscillations. It achieves selective suppression of specific oscillation frequency bands and improves the system's phase margin.

[0092] S6. Using a low-frequency grid oscillator, the frequency adaptive adjustment, the weak grid stability enhancement adjustment, and the oscillation suppression adjustment are simultaneously applied to the grid angular frequency dynamic equation, and the weak grid stability enhancement adjustment and the oscillation suppression adjustment are simultaneously applied to the voltage amplitude dynamic adjustment model to determine the grid angular frequency, the grid voltage amplitude, and the grid voltage phase angle generated by the grid angular frequency, thereby generating a three-phase low-frequency grid voltage reference signal acting on the hybrid MMC.

[0093] like Figure 13 As shown, S6 specifically includes: The grid voltage amplitude is determined by using a dynamic voltage amplitude adjustment model based on the regulation amount for enhancing stability in weak grids and the regulation amount for suppressing oscillations. The grid angular frequency is determined by using the dynamic equation of grid angular frequency, based on the frequency adaptive adjustment, the weak grid stability enhancement adjustment, and the oscillation suppression adjustment. The phase angle of the grid voltage is determined based on the grid angular frequency using a grid voltage phase angle generation relationship model. Based on the phase angle and amplitude of the grid voltage, a three-phase low-frequency grid voltage reference signal acting on the hybrid MMC is generated. Specifically, the phase angle and amplitude of the grid voltage are substituted into the generation equation of the three-phase low-frequency grid voltage reference signal, and the three-phase low-frequency grid voltage reference signal of the hybrid MMC is obtained by solving the equation.

[0094] Based on the construction of a low-frequency grid oscillator and its frequency regulation and weak grid stability enhancement control, an oscillation energy function is constructed by extracting the oscillation component from the filtered AC-side active power. Adaptive adjustment of the damping coefficient is then achieved based on this oscillation energy, thus forming an independent low-frequency power oscillation suppression control channel. This channel directly introduces the oscillation state into the control structure, enabling the damping effect to be dynamically adjusted according to the oscillation intensity and its changing trend.

[0095] The constructed adaptive damping control channel suppresses the AC side power oscillation component. Synergistic damping is introduced in the frequency regulation channel and voltage regulation channel to weaken the transmission and amplification of oscillation between control links. This transforms the system damping characteristics from a fixed parameter form to an adaptive adjustment form that changes with the oscillation state, thereby effectively suppressing power oscillation during low-frequency grid rectification operation and improving the system oscillation response characteristics.

[0096] Within the framework of a unified grid oscillator, by introducing low-frequency operating characteristics, grid strength information, and low-frequency oscillation state variables, adaptive frequency regulation, weak grid stability enhancement regulation, and adaptive damping regulation for low-frequency power oscillation suppression are respectively formed. These regulation variables are then uniformly coupled and injected into the frequency dynamic equation and voltage amplitude dynamic adjustment model of the grid oscillator, enabling different control objectives to achieve coordinated adjustment within the same dynamic model. This constructs a unified and coordinated control structure distinct from existing independent control links. The system's regulation mechanism is transformed from the existing parametric form to an adaptive regulation form that changes with the operating state. Through this control method, the frequency, voltage, and power are dynamically and coordinatedly adjusted during grid rectification operation, enhancing the system's adaptability to low-frequency operating characteristics and grid strength changes. This improves the overall operational coordination and dynamic adjustment capability of the hybrid MMC during grid rectification operation under low-frequency weak grid conditions.

[0097] The following section, based on experiments, explains the beneficial effects of the adaptive rectification control method for hybrid MMC low-frequency weak grid oscillators.

[0098] The main observations of this experiment are the AC side output frequency, AC side voltage, AC side current, AC side input power, DC side output power, internal circulating current, and submodule capacitor voltage of the converter, which are used to verify the effectiveness of this method.

[0099] After the hybrid MMC starts up, it first generates a three-phase low-frequency grid voltage reference signal based on the constructed low-frequency grid oscillator, and then forms a converter modulation command based on this signal to realize the active establishment of AC side voltage. Figure 2 The waveform of the network frequency (f) response is given. Figure 2As can be seen, under the proposed method, the system can quickly establish the target low-frequency operating frequency, and the frequency adjustment process is smooth without significant continuous oscillations. This indicates that the constructed low-frequency grid oscillator can achieve stable grid construction under low-frequency weak grid conditions. Simultaneously, the introduction of adaptive frequency adjustment allows the frequency adjustment capability to automatically adjust with changes in the system's operating state, thereby ensuring that the grid frequency maintains good response speed and stability under disturbance conditions.

[0100] Once the DC-side power demand is established, the converter enters the grid-connected rectification operation state. Figure 3 The DC side voltage (U) is given. dc Waveform diagram. (From...) Figure 3 As can be seen, the DC-side voltage remains stable during rectification operation without significant overshoot or continuous fluctuations, indicating that this method can maintain DC-side voltage stability while absorbing power from the AC side and transferring energy to the DC side. This result demonstrates that grid control and rectification control are not independent but rather achieve coordinated control under the combined action of a unified grid oscillator and integrated regulation.

[0101] Figure 4 The AC side voltage (U) is given. AC Waveform, Figure 5 The locally amplified waveform of the AC side voltage is given. Figure 6 The AC side current (I) is given. AC Waveform, Figure 7 The locally amplified waveform of the AC side current is given. Figure 4 , Figure 5 and Figure 6 , Figure 7 It is evident that during grid-connected rectification operation, the AC side voltage waveform is stable and the current waveform is continuous, reflecting the normal power absorption process of the converter under low-frequency operating conditions. Especially under weak grid conditions, the voltage and current maintain good dynamic characteristics, indicating that introducing weak grid stability enhancement regulation can effectively improve the system's voltage support capability and enhance the converter's adaptability to changes in grid conditions.

[0102] Figure 8 The AC side input power (P) is given. ac )curve, Figure 9 The DC-side output power (P) is given. dc ) curve. From Figure 8 and Figure 9It is evident that during grid-connected rectification operation, the AC input power and DC output power maintain a good dynamic tracking relationship, with smooth power changes and no obvious continuous oscillations or amplification phenomena. This result indicates that, under the combined action of the low-frequency grid oscillator, the frequency adaptive regulation, and the weak grid stability enhancement regulation, the system can maintain good dynamic stability during power transmission. Furthermore, introducing an oscillation suppression regulation can weaken the low-frequency oscillations that may occur during power changes, thereby improving the power dynamic response characteristics under grid-connected rectification operation conditions.

[0103] Figure 10 The internal circulation of hybrid MMC (I) is given Larm Waveform. (From) Figure 10 As can be seen, under the proposed method, the internal circulating current fluctuations of the converter remain within a small range, without significant oscillation amplification, indicating that the system exhibits good overall stability during dynamic frequency, voltage, and power regulation. Furthermore, this result demonstrates that the unified grid oscillator and multi-source coordinated regulation mechanism constructed using this method do not introduce additional internal circulating current instability factors during dynamic operation.

[0104] Figure 11 The submodule capacitor voltage (U) is given. cap ) change curve, Figure 12 A partially enlarged view of the capacitor voltage of the submodule is provided. Figure 11 and Figure 12 As can be seen, the overall voltage variation trend of each submodule's capacitor is consistent, and the voltage fluctuation remains within a small range. Even after local magnification, the voltage deviation between submodules is still small, indicating that under the action of this method, the energy distribution inside the hybrid MMC is relatively balanced, and the submodules operate stably. This result further illustrates that the coupling effect between the grid oscillator, dynamic voltage regulation, weak grid enhancement, and oscillation damping control can provide favorable operating conditions for each submodule within the MMC.

[0105] comprehensive Figures 2 to 12 It can be seen that the hybrid MMC in this method can achieve stable grid-connected rectification operation under low-frequency weak grid conditions. The system can actively establish low-frequency AC side voltage and frequency, and maintain stable voltage amplitude and frequency changes during operation, while the DC side voltage and output power remain stable.

[0106] During dynamic operation, the AC power change was smooth, without any obvious continuous oscillation or amplification, indicating that the system has good power dynamic response characteristics under low-frequency operating conditions. At the same time, the various state variables inside the converter (including the arm current and submodule capacitor voltage) operated stably, without any abnormal fluctuations or instability.

[0107] This demonstrates that the proposed method can achieve stable frequency and voltage regulation under low-frequency weak power grid conditions and ensure the dynamic stability of the overall system operation, thus verifying the feasibility and effectiveness of the proposed method.

[0108] In summary, this method uses a low-frequency grid oscillator as the core control unit for hybrid MMC grid rectification control. This oscillator uniformly generates the grid voltage phase angle, grid angular frequency, and grid voltage amplitude. Frequency adaptive adjustment, weak grid stability enhancement adjustment, and oscillation suppression adjustment are introduced into the grid angular frequency dynamic equation to determine the grid angular frequency. Weak grid stability enhancement and oscillation suppression adjustment are introduced into the voltage amplitude dynamic adjustment model to determine the grid voltage amplitude. The grid voltage phase angle is determined based on the grid angular frequency through the grid voltage phase angle generation relationship model. These adjustment quantities act on the same grid oscillator model through a unified interface, so that frequency regulation, voltage support, and power oscillation suppression are no longer independent control links, but form a coordinated adjustment relationship within the same dynamic model. This structure can better adapt to frequency changes, grid strength changes, and power oscillation states under low-frequency weak grid rectification operation conditions, improving the overall system control consistency and dynamic adjustment capability.

[0109] This method constructs a unified descriptive framework for the frequency dynamic equation and the voltage amplitude dynamic adjustment model. It introduces adjustment components such as power deviation, frequency deviation, phase angle deviation, adaptive frequency adjustment, weak grid stability enhancement adjustment, and oscillation suppression adjustment into the model, enabling the generation process of the grid voltage to dynamically change with the system's operating state. This method breaks through the existing independent control mode of frequency and voltage regulation in grid control, achieving unified dynamic adjustment between grid voltage and frequency, thereby improving the system's dynamic response capability and stability under complex operating conditions.

[0110] This method introduces a low-frequency operating factor and uses it as a crucial input for adaptive frequency regulation in the dynamic frequency adjustment channel of a low-frequency gridded oscillator, establishing a relationship between the low-frequency operating factor and the frequency regulation gain. Through this mechanism, the adaptive frequency adjustment not only balances the requirements of dynamic frequency response speed and system stability but also adaptively adjusts with changes in the system operating frequency, ensuring that the gridded angular frequency generation process matches the low-frequency operating state. This improves the adaptability and dynamic regulation performance of the frequency response under low-frequency gridded rectification operation conditions.

[0111] This method estimates the equivalent impedance of the power grid to construct a power grid strength characterization quantity, and then forms a weak grid stability enhancement regulation quantity based on this quantity. Simultaneously, it extracts the low-frequency power oscillation component on the AC side to construct an oscillation energy function, and then forms an oscillation suppression regulation quantity based on this energy. These weak grid stability enhancement and oscillation suppression regulation quantities are not applied separately to independent control loops, but are uniformly introduced as regulation components into the frequency dynamic equation and voltage dynamic equation of the low-frequency grid oscillator. This achieves coordinated regulation of voltage support enhancement and power oscillation suppression within the same grid control model, improving the dynamic stability and operational adaptability of the system under low-frequency weak grid rectification operation conditions.

[0112] Finally, it should be understood that the embodiments described in this specification are merely illustrative of the principles of the embodiments described herein. Other variations may also fall within the scope of this specification. Therefore, alternative configurations of the embodiments described herein are intended to be illustrative rather than limiting, and should be considered consistent with the teachings of this specification. Accordingly, the embodiments described herein are not limited to those explicitly introduced and described herein.

Claims

1. An adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator, characterized in that, include: Constructing a low-frequency grid oscillator, wherein the construction of the low-frequency grid oscillator includes a grid voltage phase angle generation relationship model, a grid angular frequency dynamic equation, and a voltage amplitude dynamic adjustment model; Collect status data of the hybrid MMC, wherein the status data includes at least AC side active power, reactive power, DC side power demand, and voltage and current at the low-frequency AC side connection point of the converter; The active power on the AC side is filtered, and the deviation of the active power on the AC side after filtering is obtained by combining the power demand on the DC side. The low-frequency operating factor, adaptive frequency regulation gain and frequency regulation dynamic compensation amount are calculated to determine the adaptive frequency regulation amount. Based on the current and voltage changes at the low-frequency AC side connection point of the converter, the equivalent short-circuit ratio estimate and the sensitivity of the weak grid are calculated. Combined with the voltage deviation, reactive power deviation, voltage change rate and the voltage dynamic compensation of the weak grid at the low-frequency AC side connection point of the converter, the stability enhancement regulation of the weak grid is determined. The low-frequency oscillation power component is extracted from the active power on the AC side after filtering, and the oscillation power is obtained by bandpass filtering. Based on the oscillation power, the oscillation energy function is constructed, the adaptive damping adjustment coefficient is calculated, and the oscillation suppression adjustment amount is determined by combining the dynamic compensation amount of oscillation power. By using a low-frequency grid oscillator, the frequency adaptive adjustment, the weak grid stability enhancement adjustment, and the oscillation suppression adjustment are simultaneously applied to the grid angular frequency dynamic equation, and the weak grid stability enhancement adjustment and the oscillation suppression adjustment are simultaneously applied to the voltage amplitude dynamic adjustment model to determine the grid angular frequency, the grid voltage amplitude, and the grid voltage phase angle generated by the grid angular frequency, thereby generating a three-phase low-frequency grid voltage reference signal that acts on the hybrid MMC.

2. The adaptive rectification control method for hybrid MMC low-frequency weak grid oscillator according to claim 1, characterized in that, The dynamic equation for the angular frequency of the network is: ， ， in, For virtual inertia coefficient, The rate of change of the angular frequency of the network is This is a reference value for active power. For AC side active power, This is the frequency damping coefficient. For the angular frequency of the mesh, For the target low-frequency angular frequency, This is the phase angle synchronization adjustment coefficient. For the grid voltage phase angle, The equivalent phase angle of the power grid, For comprehensive adjustment, This is the frequency adaptive adjustment amount. To enhance regulation in weak power grids, This is the oscillation suppression adjustment amount.

3. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to claim 1, characterized in that, The voltage amplitude dynamic adjustment model is as follows: ， in, The voltage regulation time constant, The rate of change of the grid voltage amplitude, This serves as a reference value for the grid voltage amplitude. This represents the equivalent amplitude of the AC-side grid voltage. This is the reactive power-voltage regulation coefficient. For AC side reactive power, This is a reference value for reactive power. These are the weighting coefficients. To enhance regulation in weak power grids, This is the damping injection coefficient for the oscillation suppression regulation on voltage amplitude adjustment. This is the oscillation suppression adjustment amount.

4. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to claim 1, characterized in that, The active power on the AC side is filtered, and the deviation of the filtered active power on the AC side is obtained by combining it with the power demand on the DC side. The low-frequency operating factor, adaptive frequency regulation gain, and frequency regulation dynamic compensation are calculated to determine the adaptive frequency regulation amount, including: The active power on the AC side is dynamically filtered to generate filtered active power on the AC side. Based on the reference power obtained by mapping the DC side power demand and the filtered AC side active power, a filtered AC side active power deviation is generated. Based on the filtered AC-side active power deviation, the dynamic characteristic quantity and integral state quantity of the filtered AC-side active power deviation are calculated. Calculate the low-frequency operating factor based on the reference angular frequency and the angular frequency output by the grid oscillator; Calculate the adaptive frequency adjustment gain based on the low-frequency operating factor; Based on the filtered AC-side active power deviation and the dynamic characteristic quantity of the filtered AC-side active power deviation, the dynamic compensation quantity of frequency regulation is calculated. The adaptive frequency adjustment is calculated based on the filtered AC-side active power deviation, the dynamic characteristic quantity and integral state quantity of the filtered AC-side active power deviation, the adaptive frequency adjustment gain, and the frequency adjustment dynamic compensation quantity.

5. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to claim 4, characterized in that, The frequency adaptive adjustment amount is calculated based on the following formula: ， in, This is the frequency adaptive adjustment amount. To adaptively adjust the gain at the frequency, , , and These are the weighting coefficients. This refers to the AC-side active power deviation after filtering. This is the dynamic characteristic quantity of the AC-side active power deviation after filtering. This is the integral state quantity of the AC-side active power deviation after filtering. This is the dynamic compensation amount for frequency adjustment; After introducing the frequency adaptive adjustment, the system frequency adjustment channel is as follows: ， ， in, For the Laplace transform of the angular frequency deviation of the network, This is the Laplace transform of the filtered AC-side active power deviation. For the Laplace operator, This is the phase angle synchronization adjustment coefficient. For frequency adaptive gain adjustment, For virtual inertia coefficient, This is the frequency damping coefficient. The transfer function of the composite adjustment structure, , The lead compensation time constant, , This is the lag adjustment time constant.

6. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to claim 5, characterized in that, Based on the voltage and current changes at the low-frequency AC side connection point of the converter, the equivalent short-circuit ratio estimate and the sensitivity of the weak grid are calculated. Combined with the voltage deviation, reactive power deviation, voltage change rate at the low-frequency AC side connection point of the converter, and the dynamic voltage compensation of the weak grid, the stability enhancement regulation quantities for the weak grid are determined, including: The equivalent impedance of the power grid is calculated based on the current and voltage changes at the low-frequency AC side connection point of the converter using the estimation model of the equivalent impedance of the power grid. Calculate the equivalent short-circuit ratio estimate based on the power grid equivalent impedance; Calculate the sensitivity of the weak power grid based on the equivalent short-circuit ratio estimate; Calculate the dynamic compensation amount for weak grid voltage based on the voltage deviation and voltage change rate at the low-frequency AC side connection point of the converter. The regulation amount for enhancing stability of the weak power grid is calculated based on the voltage deviation, reactive power deviation, voltage change rate, and weak power grid sensitivity at the low-frequency AC side connection point of the converter.

7. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to claim 6, characterized in that, The stability enhancement regulation for weak power grids is calculated based on the following formula: ， in, To enhance regulation in weak power grids, To enhance adaptive gain for weak grid stability, , , and These are the weighting coefficients. This refers to the voltage deviation at the low-frequency AC side connection point of the converter. This refers to reactive power deviation. The rate of change of voltage. This is the dynamic voltage compensation amount for weak power grids; After introducing frequency adaptive regulation and weak grid stability enhancement regulation, the system frequency regulation channel is represented as follows: ， ， in, The equivalent transfer function for the stability enhancement channel in a weak power grid is... To enhance adaptive gain for weak grid stability, This is the advance compensation time constant; The main dynamic time constant; is the filter damping time constant.

8. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to claim 7, characterized in that, The low-frequency oscillation power component is extracted from the filtered AC active power, and the oscillation power is obtained after bandpass filtering. An oscillation energy function is constructed based on the oscillation power, the adaptive damping adjustment coefficient is calculated, and the oscillation suppression adjustment is determined by combining the dynamic compensation amount of the oscillation power, including: The average power component is calculated based on the filtered AC active power. The low-frequency oscillation power component is calculated based on the filtered AC side active power and average power component. The bandpass filtered oscillation power is generated based on the low-frequency oscillation power component using a second-order bandpass filter. Calculate the oscillation energy based on the oscillation power after bandpass filtering; Calculate the adaptive damping adjustment coefficient based on the oscillation energy; Calculate the dynamic compensation amount of oscillation power based on the low-frequency oscillation power component and the rate of change of the low-frequency oscillation power component; The oscillation suppression adjustment amount is determined based on the dynamic compensation amount of oscillation power, the adaptive damping adjustment coefficient, the low-frequency oscillation power component, and the rate of change of the low-frequency oscillation power component.

9. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to claim 8, characterized in that, The oscillation suppression adjustment amount is calculated based on the following formula: ， in, This is the oscillation suppression adjustment amount. , and These are the weighting coefficients. This is the adaptive damping adjustment coefficient. This is the low-frequency oscillation power component. The rate of change of the low-frequency oscillation power component. This is the dynamic compensation amount for oscillation power; After introducing frequency adaptive adjustment, weak grid stability enhancement adjustment, and oscillation suppression adjustment, the system frequency adjustment channel is expressed as follows: ， ， in, This is the adaptive damping adjustment coefficient. This is the equivalent transfer function of the low-frequency power oscillation suppression control channel; , This is the phase lead compensation time constant. , This is the phase lag compensation time constant.

10. The adaptive rectification control method for a hybrid MMC low-frequency weak-grid oscillator according to any one of claims 1-9, characterized in that, By using a low-frequency grid oscillator, the frequency adaptive adjustment, the weak grid stability enhancement adjustment, and the oscillation suppression adjustment are simultaneously applied to the grid angular frequency dynamic equation, and the weak grid stability enhancement adjustment and oscillation suppression adjustment are simultaneously applied to the voltage amplitude dynamic adjustment model to determine the grid angular frequency, the grid voltage amplitude, and the grid voltage phase angle generated by the grid angular frequency. This, in turn, generates a three-phase low-frequency grid voltage reference signal acting on the hybrid MMC, including: The grid voltage amplitude is determined by using a dynamic voltage amplitude adjustment model based on the regulation amount for enhancing stability in weak grids and the regulation amount for suppressing oscillations. The grid angular frequency is determined by using the dynamic equation of grid angular frequency, based on the frequency adaptive adjustment, the weak grid stability enhancement adjustment, and the oscillation suppression adjustment. The phase angle of the grid voltage is determined based on the grid angular frequency using a grid voltage phase angle generation relationship model. Based on the phase angle and amplitude of the grid voltage, a three-phase low-frequency grid voltage reference signal is generated that acts on the hybrid MMC.

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